Chemical vapor deposition (CVD) is a one-step, vacuum process, involving no solvents or volatiles. Using CVD, monomers are converted directly to desired polymeric films without the need for purification, drying, or curing steps. Custom copolymers can be created simply by changing the ratio of feed gases to the CVD reactor (Murthy, S. K.; Gleason, K. K. Macromolecules 2002, 35, 1967). CVD allows films of nanoscale thicknesses with macroscale uniformity to be produced and can be applied to complex geometries (Pierson, H. O. Handbook of Chemical Vapor Deposition, 2nd ed.; Noyes Publications: Norwich, N.Y., 1999). CVD can also be used to coat nanoscale features, as there are no surface tension and non-uniform wetting effects that are typically associated with wet processes. Plasma-enhanced CVD (PECVD) is a proven method for producing PHEMA thin films (Tarducci, C.; Schofield, W. C. E.; Badyal, J. P. S. Chem. Mat. 2002, 14, 2541). In particular, the pulsation of the plasma on the μs-ms time scale has been found to allow a high degree of retention of hydroxyl groups. Systematic control of crosslink density, however, has not been demonstrated for PECVD.
Initiated CVD (iCVD) can be positioned as a complementary method to PECVD in depositing films with control of crosslink density. In contrast to PECVD, there is no plasma and hence no UV irradiation or ion bombardment during the iCVD process; therefore, the resulting films have lower densities of dangling bonds than films grown using plasma excitation (Limb, S. J.; Lau, K. K. S.; Edell, D. J.; Gleason, E. F.; Gleason, K. K. Plasmas Polym. 1999, 4, 21).
The iCVD method is a subset of hot-filament CVD (HFCVD) in which selective thermal decomposition of species is achieved using resistively-heated filament wires. The substrates to be coated are backside-cooled to promote adsorption of growth species. iCVD differs from conventional HFCVD, for example, in that an initiator in addition to the monomer is introduced into the vacuum chemical vapor deposition reactor. Mao and Gleason (Mao, Y.; Gleason, K. K. Langmuir 2004, 20, 2484) have demonstrated the iCVD of a methacrylic polymer, poly(glycidyl methacrylate) (PGMA), which is from the same chemical family as PHEMA. Glycidyl methacrylate (GMA) was the monomer, and tert-butyl peroxide (TBPO) was the initiator. Due to the weakness of the peroxy bond in TBPO, very low filament temperatures (180-250° C.) are required to generate radicals for initiation. These radicals serve as starters of polymer chains to which multiple monomer units are added. As a result of low temperatures, the bond-scission chemistry inside the chemical vapor deposition reactor is limited to the fragmentation of TBPO. The pendant epoxide groups are, therefore, preserved in the process, leading to high structural resemblance of iCVD PGMA to solution-polymerized PGMA. The use of an initiator not only allows control of chemistry, but also accelerates film growth and provides molecular-weight and rate control (Mao, Y.; Gleason, K. K. Langmuir 2004, 20, 2484; Pryce Lewis, H. G.; Caulfield, J. A.; Gleason, K. K. Langmuir 2001, 17, 7652; and Murthy, S. K.; Olsen, B. D.; Gleason, K. K. Langmuir 2002, 18, 6424). The energy input is low due to the low filament temperatures (<50 mW/cm2) and the need only to decompose the initiator, but not the monomer. Yet, high growth rates (>100 nm/min) were achieved in the iCVD of PGMA. All these benefits of iCVD position it as an improvement over conventional HFCVD, which already is a proven method for depositing poly(tetrafluoroethylene), polyoxymethylene, organosilicate glass, and fluorocarbon-organosilicon copolymer thin films (Lau, K. K. S.; Gleason, K. K. J. Fluor. Chem. 2000, 104, 119; Loo, L. S.; Gleason, K. K. Electrochem. Solid State Lett. 2001, 4, G81; Pryce Lewis, H. G.; Casserly, T. B.; Gleason, K. K. J. Electrochem. Soc. 2001, 148, F212; and Murthy, S. K.; Gleason, K. K. 2002, 35, 1967.) Radicals in iCVD processes are annihilated through termination. Both disproportionation and coupling reactions eliminate radicals and halt the addition of monomer units to the chains. The recombination of radicals avoids the presence of dangling-bond defects in the resulting polymeric film (Limb, S. J.; Labelle, C. B.; Gleason, K. K.; Edell, D. J.; Gleason, E. F. Appl. Phys. Lett. 1996, 68, 2810).
Poly(2-hydroxyethyl methacrylate) (PHEMA)
Poly(2-hydroxyethyl methacrylate) (PHEMA) and PHEMA-based materials have been of great interest and importance since their disclosure in 1960 (Wichterle, O.; Lim, D. Nature 1960, 185, 117). PHEMA-based hydrogels have been widely researched and used in biomedical applications because of their non-toxicity, non-antigenic properties, and biocompatibility (Folkman, J.; Moscona, A. Nature 1978, 273, 345). Since the ground-breaking demonstration of polymeric materials for sustained-release purposes, PHEMA and PHEMA-based materials have been investigated and used as carriers for controlled release of water-soluble drugs (Hsiue, G. H.; Guu, J. A.; Cheng, C. C. Biomaterials 2001, 22, 1763; Ferreira, L.; Vidal, M. M.; Gil, M. H. Int. J. Pharm. 2000, 194, 169; Blanco, M. D.; Trigo, R. M.; Garcia, O.; Teijon, J. M. J. Biomater. Sci.-Polym. Ed. 1997, 8, 709; Blanco, M. D.; Garcia, O.; Gomez, C.; Sastre, R. L.; Teijon, J. M. J. Pharm. Pharmacol. 2000, 52, 1319; Trigo, R. M.; Blanco, M. D.; Teijon, J. M.; Sastre, R. Biomaterials 1994, 15, 1181; Brazel, C. S.; Peppas, N. A. STP Pharma Sci. 1999, 9, 473; Garcia, O.; Blanco, M. D.; Gomez, C.; Teijon, J. M. Polym. Bull. 1997, 38, 55; Garcia, O.; Trigo, R. M.; Blanco, M. D.; Teijon, J. M. Biomaterials 1994, 15, 689; and Lehr, C. M.; Bouwstra, J. A.; Vanhal, D. A.; Verhoef, J. C.; Junginger, H. E. Eur. J. Pharm. Biopharm. 1992, 38, 55). A number of these drug-delivery studies involved the use of PHEMA and PHEMA-based thin films. PHEMA and PHEMA-based surfaces have been used for cell adhesion, cell growth, protein adsorption, separation devices, biosensors, and metal-ion adsorption (Harkes, G.; Feijen, J.; Dankert, J. Biomaterials 1991, 12, 853; Guan, J. J.; Gao, G. Y.; Feng, L. X.; Sheng, J. C. J. Biomater. Sci.-Polym. Ed. 2000, 11, 523; Lopez, G. P.; Ratner, B. D.; Rapoza, R. J.; Horbett, T. A. Macromolecules 1993, 26, 3247; Morra, M.; Cassinelli, C. J. Biomed. Mater. Res. 1995, 29, 39; Denizli, A.; Say, R.; Patir, S.; Arica, M. Y. React. Funct. Polym. 2000, 43, 17; Ibrahim, E. H.; Denizli, A.; Bektas, S.; Genc, O.; Piskin, E. J. Chromatogr. B 1998, 720, 217; Arica, M. Y.; Senel, S.; Alaeddinoglu, N. G.; Patir, S.; Denizli, A. J. Appl. Polym. Sci. 2000, 75, 1685; and Osada, Y.; Iriyama, Y. Thin Solid Films 1984, 118, 197). For micropatterning, PHEMA thin films have been demonstrated as deep-UV and e-beam resists that are developable in aqueous solutions (Vasilopoulou, M.; Boyatzis, S.; Raptis, I.; Dimotikalli, D.; Argitis, P. J. Mater. Chem. 2004, 14, 3312). Methacrylic polymers are also known to decompose thermally into small molecules, so thin-films of these materials may be used as sacrificial layers for microstructure fabrication for microelectronic and optical applications (Zaikov, G. E.; Aseeva, R. M. 1993, 74, 21; Chandra, R.; Saini, R. J. Macromol. Sci.-Rev. Macromol. Chem. Phys. 1990, C30, 155; Zulfiqar, S.; Akhtar, N.; Zulfiqar, M.; McNeill, I. C. Polym. Degrad. Stabil. 1989, 23, 299; Zulfiqar, S.; Piracha, A.; Masud, K. Polym. Degrad. Stabil. 1996, 52, 89; Zulfiqar, S.; Zulfiqar, M.; Nawaz, M.; McNeill, I. C.; Gorman, J. G. Polym. Degrad. Stabil. 1990, 30, 195).
Although PHEMA is not sufficiently hydrophilic to dissolve in water, crosslinking of the polymer is normally required to control its gel properties. For instance, the degree of crosslinking has been found to have a significant impact on the rate of drug release from PHEMA-based hydrogels. The degree of swelling has been found to decrease and the mechanical properties have been found to increase with increasing crosslink density (Lee, J. W.; Kim, E. H.; Jhon, M. S. Bull. Korean Chem. Soc. 1983, 4, 162; Perera, D. I.; Shanks, R. A. Polym. Int. 1995, 37, 133). The ability to produce thin-films of well-defined crosslink densities is therefore crucial.
Thin films of PHEMA and PHEMA-based materials are normally prepared by solution-phase grafting, casting from polymer solution, or confined solution-phase polymerization, all of which are wet processes (Zubaidi; Hirotsu, T. J. Appl. Polym. Sci. 1996, 61, 1579; Feng, M.; Morales, A. B.; Beugeling, T.; Bantjes, A.; vanderWerf, K.; Gosselink, G.; deGrooth, B.; Greve, J. J. Colloid Interface Sci. 1996, 177, 364; and Chilkoti, A.; Lopez, G. P.; Ratner, B. D.; Hearn, M. J.; Briggs, D. 1993, 26, 4825). Solution-phase grafting is a two-step process involving the creation of radicals on the surface followed by graft polymerization and requires a graftable surface. Casting requires that the polymer be soluble in a solvent, so post-treatment is necessary to create crosslinks. Confined solution-phase polymerization is able to create a crosslinked polymer thin film in one polymerization step, but the technique requires a number of solution preparation steps and subsequent confinement of the solution to produce a thin film. Although this technique allows films of different crosslink densities to be made by preparing solutions of different compositions, it is time-consuming and has poor thickness control.
In contrast to these wet techniques, an all-dry process might be used to produce thin-film coatings on materials that would otherwise dissolve in solvents used in wet processes (e.g., drug particles). A dry process would also offer environmental benefits by mitigating the use of solvents (e.g., N,N-dimethylformamide) and avoiding potential retention of solvents in the films. The release of drugs from hydrogels typically involves gel formation in the presence of dissolved drugs in the polymerization solution or post-polymerization swelling of the gel to incorporate drugs within it. An all-dry process would allow coating of pre-manufactured drug particles for controlled release. Such a coating would act as a membrane that swells in water, and the diffusional transport of drugs would depend on the thickness and the crosslink density.